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R: SNR measurements



Lee KU4OS,

The metod that i use for (S+N)/N  measurements on sun noise or any other
signal at the output of my receiver is quite easy and accurate:

1)Switch OFF the AGC of your receiver

2)Switch the receiver to SSB or CW wich works linearly using a product
   detector

3)Reduce to a minimum the RF gain control (all CCW)

4)Increase to maximum the audio gain control (all CW)

5)Connect an AC voltmeter 5 volt f.s to the audio output (best high
   impedance)

6)Put your antenna to the cold sky  and increase the RF gain until you get 1
   volt AC indication and call this voltage V2 (this is your noise reference
   baseline)

7)Without changing any adjustements put your antenna to the sun and read the
   new indication and call this voltage V1

Your  (S+N)/N =  20 log  (V1/V2)
                                        10

For example, if  V1 reading is 1.42 volts than your (S+N)/N = 3 dB

Remember that your S meter or voltmeter are measuring (S+N)/N and not S/N
and for this concept i recommend to read "On Decibel and Noise" by W3GKP
published  in QST january 1968  pages 34 to 36

If you want to convert (S+N)/N in to S/N use the formula S/N= (S+N/N)-1
where (S+N)/N is in factor and not dB

For the above example (S+N)/N = 3 dB = 2 in factor
and S/N= 2-1 =1 or 0 dB

So when you are measuring 3 dB of (S+N)/N your S/N is 0 dB or signal=noise

Increasing the value of  (S+N)/N the difference between (S+N)/N and S/N
became smaller

For example if (S+N)/N= 10 dB than S/N= 9.54 dB

For measurements of  (S+N)/N on AO40 MB or any constant carrier in to the
passband,tune the audio output for maximum V1 using the above procedure
with Rx in SSB or CW and make the same calculations as above.

The above measurement is accurate because signal and noise are measured in
to the same passband of the receiver IF

In order to simplify the above measurement and calculations i have built a
linear dB meter using a RMS to DC chip Converter AD 636 from Analog Device
and i have linearly calibrated for it a 100 uA meter from 0 dB to 20 dB

If you or any other is interested in this matter i can send as an
attachement both the QST article and the dB meter diagram wich is actually
widely used by the EME operators.

W0LMD Robert is using as well the above instrument with a chip AD 636

73" de i8CVS Domenico

----- Original Message -----
From: Lee <lee-fl@cfl.rr.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, February 17, 2003 5:05 PM
Subject: [amsat-bb] SNR measurements


> I've been trying to make signal to noise measurements on AO-40 over the
> weekend and seem to be getting numbers better than I would expect.  As a
> result I'm inclined to believe that my technique is in error since sun &
> ground noise measurements result in fairly reasonable values for system NF
> and gain.  What would be the best approach or software to use for making
SNR
> measurements?  I would particularly like to find a technique for measuring
> the SNR of the MB's BPSK signal since that eliminates a lot of uplink
> related variables.
>
> Using Spectran, I tried measuring from the null in the center of the
middle
> beacon to the peak during the idle period between blocks and get a value
of
> 30 dB when the transponder is on and 33 dB during the mode-K window from
MA
> 124-126.  While I suspect the absolute numbers, the change is consistent
> with the drop in noise floor that I see when the receivers are
disconnected
> from the S2 tx.
>
> Following the mode-K break I looked at CW carriers at two power levels.
14w
> EIRP resulted in Spectran showing a carrier peak 11 dB above the noise
> floor.  Increasing the power to 140w EIRP resulted in an increase of the
> peak to 16 dB. During the measurements the U-Rx AGC varied from 6 to 13 dB
> with an average value of 10 dB.
>
> Comments and suggestions would be greatly appreciated.
>
> 73,
> Lee-KU4OS
>
> ----
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